I guess you need the HTPC because your TV has no apps. OMV is not designed to supply an HTPC. My advice is to connect an android stick or a raspberry to the TV, in this way you can access the services offered by the server.
If that's the case.. You can get a Firestick for next to nothing and App yourself to death if you like... then separate your NAS and HTPC the way they should be. Or as you said, a Pi/Android TV box.. both of those can be had fairly cheap (you have to watch some of those cheap china brand Android TV boxes.. make sure the box actually has Android TV). Could also go balls out, and get a Shield. I love my Shield. If you get a quality box or Firestick, rather than some cheap China brand... it will let you watch Prime/Netflix, etc. in HD.. where most will restrict the cheaper units to SD... if that's a big deal to you